The invention relates generally to dye transfer or thermal printers, and in particular to the problem of crease or wrinkle formation in successive dye transfer areas of a dye donor web. Crease formation in the dye transfer area can result in an undesirable line artifact being printed on a dye receiver.
A typical multi-color dye donor web that is used in a dye transfer or thermal printer is substantially thin and has a repeating series of three different rectangular-shaped color sections or patches such as a yellow color section, a magenta color section and a cyan color section. Also, there may be a transparent colorless laminating section immediately after the cyan color section.
Each color section of the dye donor web consists of a dye transfer area that is used for dye transfer printing and a pair of opposite longitudinal edge areas alongside the dye transfer area which are not used for printing. The dye transfer area is about 152 mm wide and the two edge areas are each about 5.5 mm wide, so that the total web width is approximately 163 mm.
To make a multi-color image print using a thermal printer, a motorized donor web take-up spool pulls the dye donor web from a donor web supply spool in order to successively draw an unused single series of yellow, magenta and cyan color sections over a stationary bead of selectively heated resistive elements on a thermal print head between the two spools. Respective color dyes within the yellow, magenta and cyan color sections are successively heat-transferred, via the bead of selectively heated resistive elements, in superimposed relation onto a dye receiver medium such as a paper or transparency sheet or roll, to form the color image print. The bead of resistive elements often extends across the entire width of a color section, i.e. across its dye transfer area and the two edge areas alongside the dye transfer area. However, in this instance, only those resistive elements that contact the dye transfer area are selectively heated. Those resistive elements that contact the two edge areas are not heated. In other words, the dye transfer is effected from the dye transfer area to the dye receiver medium, but not from the two edge areas to the dye receiver medium.
As each color section, including its dye transfer area and the two edge areas alongside the dye transfer area, is drawn over the bead of selectively heated resistive elements, the color section is subjected to a longitudinal tension particularly by a forward pulling force of the motorized donor web take-up spool. Since the dye transfer area is heated by the resistive elements, but the two edge areas alongside the dye transfer area are not, the dye transfer area is significantly weakened and therefore vulnerable to stretching as compared to the two edge areas. Consequently, the longitudinal tension will stretch the dye transfer area relative to the two edge areas. This stretching causes the dye transfer area to become thinner than the non-stretched edge areas, which in turn causes some creases or wrinkles to develop in the dye transfer area, mostly in those regions of the dye transfer area that are close to the two edge areas. The creases or wrinkles occur mostly in the regions of the dye transfer area that are close to the two edge areas because of the sharp, i.e. abrupt, transition between the weakened transfer area and the stronger edge areas. Moreover, they tend to be slanted diagonally across such regions of the dye transfer area.
As the dye donor web is pulled by the motorized donor web take-up spool over the bead of selectively heated resistive elements, the creases or wrinkles tend to spread from a trailing or rear end portion of a used dye transfer area at least to a leading or front end portion of the next dye transfer area to be used. A problem that can result is that the creases or wrinkles in the leading or front end portion of the next dye transfer area to be used will cause undesirable line artifacts to be printed on a leading or front end portion of the dye receiver medium, when the dye transfer occurs at the creases in the leading end portion of the next dye transfer area to be used. The line artifacts printed on the dye receiver medium are relatively short, but quite visible.
The question presented therefore is how to solve the problem of the creases or wrinkles being created in an unused transfer area so that no line artifacts are printed on the dye receiver medium during the dye transfer.
The Cross-Referenced Applications
The cross-referenced applications disclose a thermal printer capable of preventing slanted crease formation in a dye transfer area of a dye donor web that can cause line artifacts to be printed on a dye receiver during a dye transfer from the dye transfer area to the dye receiver.
To prevent slanted crease formation, cross-referenced application Ser. No. 10/392,502 discloses a pair of conical-shaped web-spreading rollers positioned to extend diagonally across at least the regions of the dye transfer area in which there can be slanted crease formation. The web-spreading rollers oppose any crease formation in such regions by urging the regions to spread.
On the other hand, cross-referenced application Ser. No. 10/394,888 discloses a single web-spreading roller on which fibers are diagonally wound approximately 45xc2x0 inwardly towards one another from coaxial opposite ends of the roller. The diagonal fibers spread the regions of the dye transfer area in which there can be slanted crease formation, to oppose such crease formation.
A thermal printer capable of preventing crease formation in a dye transfer area of a dye donor web that can cause line artifacts to be printed on a dye receiver during a dye transfer from the dye transfer area to the dye receiver, said printer comprising:
a thermal print head for heating the dye transfer area of the dye donor web sufficiently to effect a dye transfer from the dye transfer area to the dye receiver, but not heating two opposite edge areas of the dye donor web alongside the dye transfer area sufficiently to effect a dye transfer from the two edge areas to the dye receiver, so that the dye transfer area is vulnerable to being stretched relative to the two edge areas;
a donor web take-up that exerts a pulling force on the dye transfer area and two edge areas at the print head which longitudinally tensions the dye transfer area and two edge areas, to tend to cause the dye transfer area to stretch relative to the two edge areas, to possibly form slanted creases extending at least across respective regions of the dye transfer area adjacent the two edge areas; and
a crease-preventing web roller having respective helical grooves spiraled inwardly from coaxial opposite ends of the roller to form resilient helical ribs that, when deformed towards the opposite ends because of the longitudinal tensioning of the dye transfer area and two edge areas, cause at least the regions of the dye transfer area in which the slanted creases can form to spread in opposition to crease formation, so that line artifacts will not be printed.